Rotary head assembly for magnetic recording and reproducing device

ABSTRACT

The present invention relates to a rotary head assembly for a magnetic recording and reproducing device such as VTR. A stationary center shaft is mounted on the side of a stationary cylinder while a rotary sleeve which is fitted over said stationary center shaft for rotation about the axis thereof is disposed on the side of a rotary cylinder. A lubricating oil is injected between the rotary sleeve and the stationary center shaft so that a space between the cylinders may be utilized so as to provide a hydraulic bearing portion. Thus, the rotary head assembly may be made compact in size and to a higher degree of accuracy. In addition, the thrust load carrying point of the hydraulic bearing portion is located on the side of the rotary cylinder, and the portion adjacent to the thrust load carrying point has a sealing arrangement or construction so as to prevent the leakage of lubricating oil and to stabilize the rotation of the rotary cylinder.

BACKGROUND OF THE INVENTION:

The present invention relates to a rotary head assembly for a magneticrecording and reproducing device or apparatus and more particularly to arotation transmission means or device such as a rotary head cylinderassembly for video tape recorders (to be referred to as "VTR" forbrevity in this specification) which requires a high speed and highlyaccurate transmission of rotation.

The problems imposed on the mechanical system, especially on the rotaryhead cylinder assembly of VTR are in general how to correctly maintainthe relative speed and position between the head and the tape and how toreproduce. When there exist the variations in relative speed between thetape and head, the time axis of reproduced signal varies, causing thevariations in hue and saturation of reproduced images and thehorizontally off center raster. It follows therefor that the variationsin relative speed must be reduced to a minimum.

In general, the rotation transmission devices must satisfy the followingrequirements:

1. the deviations in rotational speed must be reduced to a minimum; and

2. radial vibration must be reduced to a minimum.

However, the prior art rotary head assemblies are in general such thatthe upper cylinder is supported by a pair of spaced rolling contactbearings. As a result, the variation in amount of lubricant (which is ingeneral grease) sealed in the rolling-contact bearings and the methodsfor sealing the lubricant are immediately reflected in the variations intorque. That is, the variations in load are increased with increase inamount of lubricant, but when the lubricant is not sufficient, rapidwear results. Furthermore, unsatisfactory running accuracy ofrolling-contact bearings also causes the variations in torque.

There has been an increasing demand for VTR which is both handy andportable. To this end, the rotary head assemblies must be made compactin size. Then, the inertia effect of the rotary head cylinder is reducedso that the variations in torque and consequently the variations inrotational speed result.

Vibration of the rotary head assembly is also caused by unsatisfactoryrunning accuracy of rolling-contact bearings. Excessive play betweenmating parts also causes vibration. Because of vibration, the center oraxis of rotation is deviated, resulting in the variations in rotationalspeed. As described above, the variations in rotational speed adverselyaffect the quality of reproduced images in the case of the rotary headassembly for VTR.

The problems of vibration and variations in rotational speed ariseunavoidably as long as the rolling-contact bearings are used. Thereforethere have been made various attempts to overcome these problems. Forinstance in order to reduce the vibration of the upper cylinder, thecomponent parts of rolling contact bearings and rotary head assembly aremachined and assembled at a higher degree of accuracy, and the moment ofinertia of the rotary shaft is increased. However, the reduction in sizeof the rotary head assembly and hence VTR results in the decrease inflywheel effect of the upper cylinder. As a result, running accuracy ofrolling contact bearings becomes more and more an important factor whichaffects vibration and variations in rotational speed of the uppercylinder.

Because of the disadvantages of the rolling contact bearings, there havebeen made various attempts to use pneumatic bearings in the rotary headassemblies for VTR especially for industrial or professional use.However, the pneumatic bearings are disadvantageous in that it requiresa pressurized air source; that is, an air compressor and a pneumaticcontrol circuit which is very complicated in construction. Therefore, itis difficult at present to incorporate the pneumatic bearings in therotary head assemblies for home VTR which must be made portable.

In addition to the problems of vibration and variations in rotationalspeed, the use of rolling contact bearings gives rise to anotherproblem. That is, in order to provide portable VTR, the dimensions ofVTR must be reduced as practically as possible. The height of portableVTR is dependent on the height of the rotary head assembly which in turnis dependent upon the distance between a pair of rolling contactbearings supporting the rotary shaft of the upper cylinder. Thus, aslong as the rolling contact bearings are used in the rotary headassemblies in VTR, there exists a limit to the reduction in size ofportable VTR.

SUMMARY OF THE INVENTION

Accordingly, one of the objects of the present invention is to provide arotary head assembly for a magnetic recording and reproducing devicewhich may ensure a higher degree of accuracy in rotation and may be madecompact in size and simple in construction so as to facilitate the massproduction of rotary head assemblies.

Briefly stated, to this end, the present invention provides a rotaryhead assembly for a magnetic recording and reproducing device of thetype comprising a stationary center shaft one end of which is securelyheld in position, a stationary cylinder having a cylindrical surfacecoaxial with said stationary center shaft, a rotary cylinder disposedadjacent to one end of said stationary cylinder for rotation about theaxis of said stationary center shaft and having a cylindrical surfacecoaxial and coplanar with that of said stationary cylinder, a means fordriving said rotary cylinder, and a head mounted on said rotary cylinderin very closely spaced relationship with a tape which is threaded aroundsaid cylindrical surfaces of said stationary and rotary cylinders andtransported relative thereto, characterized in that said stationarycenter shaft is held stationary relative to said stationary cylinder,said rotary cylinder is fitted over said stationary center shaft forrotation about the axis thereof, a rotary sleeve is fitted over saidstationary cylinder and operatively connected to said driving means, alubricating fluid is injected into the space between said rotary sleeveand said stationary center shaft so as to provide a hydraulic bearingmeans between said stationary and rotary cylinders, said hydraulicbearings means is sealed adjacent to its point carrying the thrust load,and said thrust load-carrying point is located within said rotarycylinder.

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a schematic sectional view of a prior art VTR rotary headassembly;

FIG. 2 is a schematic sectional view of a preferred embodiment of a VTRrotary head assembly in accordance with the present invention;

FIG. 3 is a fragmentary sectional view, on enlarged scale, thereof usedfor the explanation of the flow of a lubricating fluid sealed in ahydraulic bearing;

FIG. 4 is a view used for the explanation of a pivot bearing forcarrying the thrust load;

FIG. 5a is a top view of a grooved plain bearing;

FIG. 5b is a side view thereof;

FIG. 6 shows another embodiment of the present invention wherein thestationary shaft is not formed with a flange but is formed with oilgrooves so that the lubricating oil may carry thrust loads; and

FIG. 7 shows a further embodiment of the present invention which isprovided with means for preventing the leakage of lubricant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS: Prior Art, FIG. 1

Prior to the description of the preferred embodiments of the presentinvention, a prior art VTR rotary head assembly will be described inorder to more specifically point out the problems thereof.

Referring to FIG. 1, 1 denotes a rotary head cylinder (an uppercylinder) which is rotated at 1 800 rpm in case of the two-head helicalscanning type VTR. A head 3 is mounted on the rotary head cylinder 1,and a rotary shaft 4 is supported by radial bearings 5 and 6 whichreceive both the radial and thrust loads. A bushing 7 is fitted over therotary shaft 4 and is securely fitted into the rotary head cylinder 1.Sleeves 9, 10 and 11 are fitted over the rotary shaft 4 in such a waythat they press against the radial bearings 5 and 6 so as to eliminatethe axial displacement thereof.

The bearings 5 and 6 are housed within a housing 12.

The mounting of the cylinder of the type described above have thefollowing problems.

(1) The quantity of a lubricating agent (grease) sealed in the ballbearings 5 and 6 and the method of sealing the lubricant result in thevariations in torque. That is, a large quantity of lubricant or greasecauses the wide variations in load while a small quantity of lubricantresults in rapid wear. Furthermore, the undulations of the surfaces ofthe races of the bearings causes also the variations in torque.

Since the recent trend of the rotary head assemblies is toward theminiaturization, the inertia effect of the rotary cylinder head 1 isinevitably reduced so that the variations in rotation due to thevariations in torque of the bearings present serious problems.

(2) Vibrations are mainly caused by the poor surface finish andinaccuracy of component parts such as balls and outer and inner races ofthe bearings, vibrations caused by the elasticity of the outer and innerraces, and plays between the parts. Furthermore the lateral vibrationsare associated with the variations in rotational speed due to thedisplacement of the center of rotation. Thus, the variations inrotational speed and vibrations adversely affect the quality of thepictures reproduced.

The above problems (1) and (2) arise inevitably because of the use ofball bearings. In the design of head assemblies, various efforts havebeen made so as to overcome these problems. For instance, in order toreduce the vibration of the cylinder to a minimum, the bearings andhousings are machined with an extremely higher degree of accuracy andthe rotary shafts are designed so as to produce a high moment ofinertia. However, due to the recent trend toward the reduction in sizeof VTR mechanisms the flywheel effect of the cylinder is more and moredecreased and the vibrations of bearings themselves present more andmore serious problems.

In addition to the above described system for supporting the cylinderwith bearings, the system for pneumatically supporting the cylinder hasbeen devised and used in VTR for profession use. However, the pneumaticbearings need a compressor as a source of air under pressure and apneumatic control circuit which is complicated in construction.Therefore, the adoption of this system in home VTR has been difficultbecause home VTR must be portable and simple in construction.

There has been an increasing demand for portable VTR so that thecomponent parts must be fabricated at a higher degree of accuracy andassembled compact in size. The height of VTR, especially home VTR, isdependent upon the height of the cylinder which has the followingproblems. (1) The upper cylinder 1 is of the cantilever construction sothat in case of the exchange of the head 3, the upper cylinder 1 may beeasily removed from the rotary shaft 4. Furthermore, if a bearing isdisposed above the upper cylinder, the reduction in height of the uppercylinder cannot be attained. (2) Two bearings are needed in order tosupport the rotary shaft. That is, in order to ensure the rigidity ofthe upper cylinder and the accuracy in rotation, two radial bearings areneeded. It is preferable to increase the spacing (l in FIG. 1) betweenthe bearings as practically as possible. As a result, the reduction inheight becomes difficult.

In the case of the construction shown in FIG. 1, there are three waysfor mounting a motor 101. That is,

(1) the motor may be interposed between the upper cylinder 1 and theupper bearing 6;

(2) between the upper and lower bearings 6 and 5; and

(3) below the lower bearing 5 (as actually shown in FIG. 1).

As shown in FIG. 1, it is preferable to locate rotary transformers 29and 30, which operate on very weak head signals, as closely aspractically possible to the undersurface of the upper cylinder 1.

However, when the motor is located between the upper cylinder 1 and theupper bearing 6, it must be positioned between the rotary transformer 30and the upper bearing 6. As a result, the shaft for supporting the uppercylinder 1 in a cantilever manner is increased in length andconsequently the rigidity of rotating parts decreases.

When the motor is interposed between the upper and lower ball bearings 5and 6, separate housings must be provided. Furthermore, the alignmentbetween the upper and lower rolling contact bearings 6 and 5 becomesextremely difficult.

When the DD motor is mounted below the lower bearing 5, the stationarypart (stator) of the motor must be mounted on the lower end of thehousing while the rotor (armature) must be rigidly connected to therotary shaft 4. As a result, the over-all length of the upper cylinderassembly becomes longer so that it becomes difficult to make the uppercylinder assembly compact in size and light in weight.

In summary, with the constructions for supporting the upper cylinder 1with the rolling contact bearings, it is difficult to make the uppercylinder assembly compact in size.

THE INVENTION

FIG. 2 shows a rotary head assembly in accordance with the presentinvention. It includes an upper cylinder 13, a rotary sleeve 14 and anupper cylinder cover 15 which define a housing for the rotating side ofa hydraulic bearing. The upper cylinder cover 15 is securely mounted onthe rotary sleeve 14 with bolts 17, and an oil ring 16 is interposedbetween them in order to prevent the leakage of a lubricant. The uppercylinder 13 is removably mounted on the rotary sleeve 14 with bolts 18.

A head 19 is mounted on the upper cylinder 13 in such a way that itsextension and index angle may be fine adjusted by a head adjusting screw20. A center stationary shaft 21 is supported in a lower housing 22.

A hydraulic bearing 23 is provided by fitting a spacer on the stationaryshaft 21 inside the upper cylinder 13. The surfaces of the hydraulicbearing 23 are formed with spiral grooves by etching. The upper end ofthe stationary shaft 21 is terminated into a pivot pin 24.

A direct drive motor (DD motor) has an armature magnet 25 enclosed in amagnet case 26, a position rotor 27 and a position stator 28. The magnetcase 26 is mounted on the position rotor 27 which in turn is operativelyconnected to the rotary sleeve 14. The position rotor 27 and positionstator 28 correspond to brushes of the ordinary DC motors and have thefunction of detecting the angular position of the armature magnet 25which is a rotor.

In order to detect the signals from the head 19, there are providedfirst and second rotary transformers 29 and 30. The rotary transformer29 is mounted on the rotary sleeve 14 while the second rotarytransformer 30 and its mounting ring 31 are mounted on a stationarymember 32 and are securely bonded thereto with an adhesive.

The coil 34 of a pulse generator is mounted on the stationary member 32so as to detect the angular position of the head 19. The centerstationary shaft 21 is securely held in position by a setscrew 36screwed into the housing 22 which has a lower cylinder portion 35.

In this specification, the space which is defined between the rotarysleeve 14 and the hydraulic bearing 23 and in which are formed oil filmswill be referred to as "the hydraulic lubricating portion 37" and theannular space which is defined between the rotary sleeve 14 and thestationary shaft 21 will be defined as "the lubricant leakage preventiveportion 38". Since oil films are also formed in the lubricant leakagepreventive portion 38 so as to effect the hydraulic lubrication betweenthe rotary sleeve and the stationary shaft 21, there exists no definiteboundary between the hydraulic lubricating portion 37 and the lubricantleakage preventive portion 38.

FIG. 3 shows, on enlarged scale, the spacer 100 of the hydraulicbearing. As described above, the outer surface of the spacer 100 isformed with radial spiral grooves 39. In addition, thrust spiral groovesare formed in the upper and lower surfaces 40 and 41 of a flanged disk42 extended radially outwardly from the upper end of the spacer 23.Thus, the spacer 23 has a thrust bearing portion 42 and a radial bearingportion 43.

Upon rotation of the rotary sleeve 14, the inner wall surface or themoving wall surface of the rotary sleeve 14 revolves around the outerwall surface of the stationary shaft 21 so that due to the wedge actionof the lubricant film trapped between them and the pumping action of thehydraulic spiral bearing, the pressure is generated. That is, thelubricating liquid is exerted with the forces in the directionsindicated by arrows in FIG. 3 under the pumping action because of thespiral grooves. However, when the pressures are brought intoequilibrium, the flows of the lubricating oil in the directionsindicated by the arrows will exist no longer and the lubricating liquidflows circumferentially.

The features, effects and advantages of the embodiment described abovewith reference to FIGS. 2 and 3 may be summarized as follows:

(1) The rotary head assembly may be made compact in size: That is, thefeature of the present invention is the provision of a compactarrangement.

The tendency of VTR (especially home VTR) is toward the compactness, butas described above, the height of VTR is dependent upon that of thecylinder.

As described above, when the rolling contact bearings are used in therotary head assemblies, there arise various problems. The presentinvention may substantially eliminate these problems.

For instance, in the preferred embodiment of the present invention shownin FIG. 2, the hydraulic bearing 23, which receives the thrust loads, isprovided by the use of parts located within the upper cylinder 13.

As a result, not only there may be provided an excellent rigidityagainst the loads exerted to the upper cylinder 13 (against the weightof the upper cylinder when the tape tension is directed horizontally andthe rotary head assembly is also held horizontally), but also thehydraulic bearing may be so advantageously constructed that thethickness of the upper cylinder and the rotary transformers 29 and 30become thin. For instance, in the prior art rotary head assemblies, therolling contact bearings occupy dead spaces, but according to thepresent invention, the over-all height of the cylinder assembly may bereduced at least by a degree equal to the over-all height of two rollingcontact bearings.

The thrust bearing portion 42, which is only the portion of thestationary shaft 21 extended radially outwardly or enlarged in diameter,may be located within the upper portion of the rotary sleeve 14 which inturn is located within the upper cylinder 13. As a consequence, therotary head assembly in accordance with the present invention may bemade compact in size.

The upper portion of the rotary sleeve 14 is enlarged in outer diameterso as to removably mount the upper cylinder 1 thereon while the lowerportion thereof is reduced in outer diameter so that component partssuch as those of the DD motor may be mounted thereon.

In the prior art rotary head assemblies, wherein the upper cylinder issupported by the rolling contact bearings, the outer diameters of thebearings are large so that even when the stationary center shaft 21 isused, the diameter of the sleeve into which are mounted the rollingcontact bearings becomes greater so that the mounting of component partssuch as those of the DC motor becomes difficult.

However, when the hydraulic bearing construction as shown in FIG. 2 isemployed, the wall thickness of the rotary sleeve 14 may be considerablyreduced at the lower portion and the outer diameter may be also reduced.As a result, various component parts may be easily mounted on the rotarysleeve 14. In case of the VTR cylinder to which is applied the presentinvention and as shown in FIG. 1, the component parts of the VTRcylinder assembly may be arranged very compact in size without leavingany dead space. That is, there exists no loss span which corresponds tothe distance l between the two rolling contact bearings shown in FIG. 1.

As compared with the prior art upper cylinder assembly as shown in FIG.1, the over-all length of the upper cylinder assembly in accordance withthe present invention may be reduced to one half when the DD motor samein capacity and dimensions to that mounted on the prior art uppercylinder assembly is mounted. That is,

    ______________________________________                                                   Prior Art  The Invention                                           ______________________________________                                        Over-all length                                                                            L            0.5 L                                               ______________________________________                                    

(2) An extremely higher degree of accuracy in rotational speed may beensured:

With the use of the upper cylinder assembly to which is applied thepresent invention as shown in FIG. 2, the rotary head assembly having anextremely higher degree of accuracy in rotational speed can be providedmainly because of the following three reasons:

(a) the use of the hydraulically lubricated bearing;

(b) the arrangement of the hydraulic bearing within the upper cylinder13; and

(c) the provision of the hydraulically lubricated bearing which may formoil films to a satisfactory degree. The above reasons will be describedin more detail below.

(a) Under the pressure of the films of the liquid, both the rotarysleeve 14 and the upper cylinder 13 may be rotated about the axis of thestationary shaft 20 without any mechanical contact between them.

As a result, the present invention may overcome the problems inherent tothe rolling contact bearings used in the prior art upper cylinderassemblies.

(b) In the device (FIG. 2), the hydraulic bearing is provided adjacentto the first and second rotary transformers 29 and 30 and to the uppercylinder 1 upon which is mounted the head 3 the accuracy of rotationalspeed of which must be highest. This arrangement will be described inmore detail hereinafter.

Radial vibrations of the upper cylinder 13 and/or the eccentricity ofthe upper cylinder 13 from the stationary shaft 21 cause the variationsin the relative speed between the head and the tape which in turnresults in jitters in the reproduced images. The causes responsible tothese phenomena are as follows:

(1) The upper cylinder 13 and other rotating parts are not balancedcorrectly or due to their eccentricity dynamic inequilibrium results.

(2) Eccentricity of rotating parts is caused by the tension of the tape.

(3) Oil whirls in the hydraulic bearing. How much these causes effectthe radial vibrations and eccentricity closely depends upon theconstruction of the bearing which supports the rotating part.

In the case of the cause (1), the upper cylinder 13 serves as a flywheelwhich may damp high-frequency variations in rotational speed caused bytorque ripples of the DD motor. The moment of rotation (as well as theweight and the radius of rotation) is greatest as compared with otherrotating parts.

As a result, even an extremely small out of balance of the uppercylinder 13 and/or even a very small eccentricity thereof mostly causethe radial vibrations of the cylinder 13 and consequently the head 19mounted thereon.

However, according to the preferred embodiment of the present invention,the hydraulic bearing may be located within the upper cylinder 13 asshown in FIG. 2 so that the centrifugal loads, which cause the unbalancedescribed above, may be directly born. As a result, the moment load,which tends to cause the inclination of the axis of the rotary sleeve14, will not exert on the hydraulically lubricating portion 37 withinthe upper cylinder 13. That is, an excellent dynamic equilibrium may beensured and radial vibrations may be reduced to a minimum (so thatvariations in rotational speed may be also reduced to a minimum).

Same is true for the cause (2). That is, the hydraulic lubricatingportion 37 may receive the tape tensions exerted to the upper cylinder13 in such a way that no torsional moment may be produced. As a result,the rigidity of the bearing against the radial loads may be remarkablyimproved and the eccentricity of the center of rotation of the uppercylinder may be reduced to a minimum or a negligible degree.

As to the cause (3), oil whirls results in variations in rotationalspeed corresponding to one half of a rotational speed. This is aninherent instability that the hydraulic bearings exhibit.

In order to overcome this problem, non-circular bearings have beenemployed. The present invention also makes use of a spiral groovebearing which is one of the variations of the non-circular bearings.

Since, as shown in FIG. 2, the spiral groove bearing is located inopposed relationship with the inner wall of the upper cylinder 13 andwith the first and second rotary transformers 29 and 30, the rigidity ofthe bearing due to the pumping action may be remarkably enhanced.

With the non-circular bearings such as spiral groove bearings (or thebearings with spiral grooves), the spring rigidity will not become zeroeven when the hydraulic bearing has no eccentricity because the pressureis produced so that the effects of the causes (1), (2) and (3) may be byfar reduced.

Furthermore, the provision of the pivot bearing at the upper end of thestationary shaft 21 serves to reduce the torque when the driving of theupper cylinder is started.

(c) The hydraulically lubricated bearing which may sufficient oil filmsmay be provided.

When a predetermined volume of lubricating oil is sealed in a hydraulicbearing in order to enhance the portability of an equipment, it isdifficult to attain the formation of ideal oil films because of theleakage of the lubricating oil, the air entrapped in the oil and so onexcept the lubrication systems wherein the lubricating oil is forced tocirculate from the exterior.

According to the present invention, however, the lubricating oil isalways exposed to the surrounding atmosphere at the lower end of theupper cylinder assembly; that is, the open end 43 of the rotary sleeve14. The most tightly sealed portion is located within the upper cylinder13 in which is provided the hydraulic bearing 23 and is farthest fromthe opening 43.

In the device of the present invention, the sufficient hydrauliclubrication may be attained over the inner wall surfaces of the uppercylinder 13.

When the direct drive motor as shown in FIG. 2 is used in the device ofthe present invention, the accuracy in rotational speed may be furtherimproved.

The reason is that the direct drive system which exerts no radial loaddue to the belt tension as in the belt drive system will not cause anyeccentricity of the hydraulic bearing.

Meanwhile, in this specification, the portion where the hydraulicbearing is provided or otherwise formed refers to the portion where thefilm of lubricating oil is formed.

For instance, in FIG. 2, the upper end of the pivot bearing 24 is theupper limit of the hydraulic bearing. Due to the wedge action of theviscous liquid, the hydraulic bearing is formed at the leakagepreventive portion 38.

The portions adjacent to the open end of the rotary sleeve 14 are incontact with the surrounding atmosphere so that the transition issemifluid and to boundary lubrication occurs, but there exists nodefinite boundary between the complete fluid-film or hydrodynamiclubrication described above and the boundary lubrication.

According to the present invention, when the leading end of the pivotbearing 24 is located within the inner walls of the first and secondrotary transformers 29 and 30, sufficiently high rigidity and accuracymay be attained.

(3) Excellent Oil Sealability:

One of the difficult problems encountered in the mounting of thehydraulic bearings (especially oil bearings) is how to seal thelubricant.

In case of the machine tools, the exchange and/or replenishment oflubricant may be made at any time. However, in case of portableelectrical equipment such as cylinder assemblies of VTR readilyavailable in the market, it is preferable to seal the lubricating oil insuch a way that no supply may be required.

Especially in case of the upper cylinder assemblies of VTR, which is apreferred embodiment of the present invention, the contamination ofcomponent parts from the lubricating oil must be completely avoided.

When even an extremely small quantity of oil adheres to the head 19 andthe tape, dust or the like is attracted and adheres to the oil. As aresult, the misinterpretation of the head signals results.

The result is a partial disappearance of the FM carrier, causingdrop-out noise. When the leakage of oil causes the entrainment of airinto the hydraulically lubricated bearing portion, the accuracy in therotational speed of the hydraulic bearing is adversely affected.

When the hydraulic bearings are applied to the devices and equipmentsuch as VTR cylinders which require a higher degree of accuracy inrotational speed, the entrainment of air into the hydraulicallylubricated portion causes a remarkable increase in oil whirl and aconsiderable increase in rigidity of the bearing. This fact has beenconfirmed by the extensive studies and experiments conducted by theinventors. Oil whirls causes radial vibrations and variations inrotational speed of the upper cylinder, the frequency of thesevibrations and variations being one half of a rotational speed of theupper cylinder. The decrease in rigidity of the bearing causes radialvibrations whose frequency is almost equal to a rotational speed and thedegradation of various characteristics of the device due to thedeviation from a correct position.

According to the preferred embodiment of the present invention, the factthat the radial loads of VTR cylinders (especially for home VTR) aresmall is utilized so that the present invention has succeeded inproviding an excellent construction of hydraulic bearing which mayensure the complete prevention of oil leakage for a long time.

For instance, the radial load due to the tape tension in VTR, which is apreferred embodiment of the present invention, is of the order of 50grams at the most, which is extremely low as compared with otherhydraulic bearings.

As a result, it may be said that this is a condition under which theflow to the exterior of the lubricating oil due to the pressure, whichis generated in order to establish an equilibrium with an external load,may be easily prevented.

In addition, because of the reasons to be described below, the presentinvention may attain a higher degree of oil sealability. (i) As is clearfrom FIG. 2, the rotary sleeve 14, which is formed or otherwise madeintegral with the upper cylinder 13 is in the form of an axiallyelongated pipe so that the leakage preventive portion 38; that is, anoil flow passage for preventing the leakage through the clearancebetween the rotary sleeve 14 and the stationary shaft 21 may be extendedlengthwise over a sufficiently long length.

The lubricating oil is charged into the space or clearance between therotary sleeve 14 and the stationary shaft 21 and into the hydraulicbearing 23 before the cover 15 is mounted on the upper cylinder 13.

Because of higher resistance to the flow of lubricating oil and thesurface tension provided by the oil packing 16 at the upper portion andbetween the rotary sleeve 14 and the stationary shaft 21 at the lowerportion, the oil leakage may be prevented.

The construction in accordance with the present invention ensures ahigher degree of sealability of lubricating oil so that even an oil witha low viscosity may be completely sealed.

Furthermore, since it suffices to charge the lubricating oil through theupper cylinder at a single position, the assembly and maintenance may bemuch facilitated. (ii) The upper portion (that is, the upper cylindercover 15) of the rotary sleeve 14 which is most closely located to thehead 19 and the tape may be completely sealed by a pressure-fitted type(contact type) seal such as the oil packing 16 so that the leakage ofoil after it has been charged may be avoided completely. The lower openend 43 of the hydraulic bearing in accordance with the present inventionis spaced apart from the upper cylinder 13 through the component parts25, 26 and 27 of the DD motor by a relatively long distance, the upwardspread of contamination may be substantially eliminated even when thelubricating oil leaks in a small quantity from the open end 43.

As a consequence, the component parts such as the head 19, the uppercylinder 13 and so on of the tape threading system may be maintained inclean environments.

In the preferred embodiment of the present invention, in order to reducethe torque of the driving part and to minimize the variations in loaddue to the variations in environmental temperature, a low volatile andlow viscosity ester oil (η=30 cst at 30° C.) was used, but no leakagewas observed at all over a long operating time interval.

The thrust bearing portion 42 and the upper cylinder cover 15 define ahydraulic thrust bearing between them. Since a sufficient space isavailable in the vicinity of the hydraulic thrust bearing thus defined,an excellent construction for sealing the lubricating oil may beprovided. That is, the cover 15 is securely attached through the oilpacking 16 to the rotary sleeve 14 with the bolts 17.

With this arrangement, even after the assembly, the upper cylinder 13may be easily removed from the rotary sleeve 14 (in the directionindicated by the arrow shown in FIG. 2) by loosening the setscrews 13which joins between the upper cylinder 13 and the rotary sleeve 14.Therefore the exchange of the head 19 may be much facilitated. (Afterthe use of the head for a predetermined time interval, it must beexchanged because of its wear.)

The stationary shaft 21, the rotary sleeve 14 and the upper cover 15 maybe handled as a unit so that the lubricating oil may be poured into thehydraulic bearing without causing any contamination of other componentparts with the lubricating oil. Furthermore, the assembly may besimplified.

Because of the reasons (1), (2) and (3) described above, the uppercylinder assembly for VTR which may ensure the completely tight sealingof lubricating oil and the higher accuracy in operation and may be madecompact in size may be provided.

In addition to the advantages (1), (2) and (3) described above, thepresent invention is further advantageous the upper cylinder assembliesmay be easily machined and assembled.

For instance, in the prior art upper cylinder assemblies wherein theupper cylinder is supported by the rolling contact bearings, variousproblems arise when the rotating part or the upper cylinder must berotated at a higher degree of mechanical accuracy. In case of the uppercylinder assembly shown in FIG. 1, in order to align the rolling contactbearings 5 and 6, one bearing is first mounted on the housing 12 andthen the outer race of the other bearing must be securely bonded to thehousing with a suitable adhesive.

Furthermore, in order to secure a correct coaxial relationship betweenthe upper cylinder and the rotary shaft 4 or to correctly maintain theupper surface of the upper cylinder at right angles to the axis of therotary shaft 4, a step is required for machining the bushing 7, uponwhich is mounted the upper cylinder 1, after they have been assembled.This step is cumbersome and is an obstacle to the mass production of theupper cylinder assemblies. However, the upper cylinder assembliesincorporating the hydraulic bearings in accordance with the presentinvention may substantially overcome these problems.

That is, when the component parts are fabricated with requireddimensional tolerances or accuracies, the oil film which is formeduniformly in the clearance between the rotary sleeve 14 and thestationary shaft 21 may automatically attain the correct coaxialrelationship between them. According to the present invention, thehydraulic bearing is constituted by a pair of component elements; thatis, the rotary sleeve 14 and the stationary shaft 21 so that opposed tothe prior art upper cylinder assemblies of the type shown in FIG. 1 thealignment between two rolling contact bearings 5 and 6 may beeliminated.

Furthermore, the present invention may reduce the number of componentparts of the upper cylinder assembly for VTR to a minimum. For instance,the sleeves 9, 10 and 11 which cannot be eliminated for mounting therolling contact bearings 5 and 6 in the prior art upper cylinderassembly shown in FIG. 1 may be completely eliminated.

So far the fundamental features of the construction of the uppercylinder assembly for VTR to which is applied the present invention hasbeen described, and the present invention may be equally applied tovarious fields as will be described below.

FIG. 4 shows a construction of a bearing wherein the leading end of thepivot bearing 24 is always pressed against the upper cylinder 15 so asto control the position thereof in the direction in which the thrustload is exerted.

The arrows shown in FIG. 4 show the forces exerted to the liquid whenthe present invention is applied in various modes. For instance, (1) incase of the pivot suction type (A), the thrust load carrying surface 40is formed with spiral grooves in such a manner that the liquid isexerted with the force indicated by the arrow 44.

The thrust carrying surface 41 is not needed to be formed with spiralgrooves. The clearance 45 must be sufficiently made large while theclearance 46 must be made small.

According to this system, a negative pressure is produced in theclearance 46 so that the upper cylinder 13 (the upper cylinder cover 15)is forced to press against the pivot bearing 24 because of thedifference between the negative pressure in the clearance 46 and theatmospheric pressure. The greater the area of the thrust load carryingsurface 41, the greater the suction force becomes. (2) In the case ofthe pivot suction type (B):

The thrust load carrying surfaces 40 and 41 are formed with spiralgrooves in such a way that the liquid is exerted with the forces in thedirections indicated by the arrows 47 and 48.

The dimensions of the bearing must be so determined that the clearance45 may be made as small as possible while the clearance 46 may be madeas large as possible. The pressure acting on the thrust load carryingsurface 41 is greater than that acting on the thrust load carryingsurface 40 so that the upper cylinder is exerted with the force which isdirected downwards. That is, the pivot bearing 24 and the upper cylinder15 are pressed against each other. In both the cases (A) and (B), thepivot bearing 24 may carry the thrust load.

Instead of forming the spiral grooves in the thrust load carryingsurface 41, circumferential steps may be formed.

When either of the method (A) or (B) is employed, the height of theupper cylinder 13; that is, the height of the head 19 may be determinedmechanically from the height of the leading or upper end of the pivotbearing 24. Therefore, in the case of the adjustment of the height ofthe head (that is, the position in the axial direction relative to thetape) during the assembly, it is not required to take the thickness ofoil film into consideration. Thus the adjustment is much facilitated.There has been proposed a method wherein the positive pressures areproduced in the clearances above and below the thrust bearing 42 so thatthe pivot bearing 24 is supported in a non-contact manner and the upperand lower pressures are balanced so as to support the shaft in twodirections (upper and lower directions). As compared with thisarrangement, severe dimensional tolerances of the upper and lowerclearances 45 and 46 and the thickness of the thrust bearing are notneeded.

In the cases of (A) and (B), only the tip of the pivot bearing 24 ismade into mechanical contact with the upper cylinder or upper cylindercover. Since this is a point contact, an extremely higher degree ofaccuracy in rotational speed may be attained.

The construction of the thrust bearing in accordance with the presentinvention is such that the pivot bearing 24 carries the external loadacting downwards on the upper cylinder and the hydraulic bearing carriesthe external load acting upwards on the upper cylinder. That is, thebearing carries the loads acting in two directions.

In addition, the pivot suction systems (A) and (b) have a furtheradvantage in that they may be used even when the device in accordancewith the present invention is maintained in a horizontal position or inan inverted position.

For instance, in the case of the method wherein the positive pressuresare generated in the upper and lower clearances of a thrust bearing 42,thereby maintaining the rotating part in equilibrium, the equilibriumbetween the forces is established by the weight of the rotating part,the pressure produced in the area 45 below the thrust bearing 42 and thepressure produced in the area 46 above the thrust bearing 42 so that theaxial position of the rotating part (and the height of the head 19) isdetermined.

However, when the device is maintained in an inverted position or isturned upside down, the direction of the load acting on the rotatingpart remains unchanged so that the equilibrium position supported by thefilm of oil changes.

However, when the pivot suction systems (A) and (b) are employed, thethrust bearing may be so arranged that the suction force to the pivotsurface (the vertical resistance from the upper cylinder 15) may becomegreater than the load acting on the rotating part so that the pivotbearing surface may be always maintained sucked to the upper cylindercover 15 regardless of the position of the device. Thus, a high rigidityin the axial direction may be attained. As a result, the absolute heightof the head 19 remains unchanged.

The greater the diameter of the flange of the thrust bearing 42, thegreater the suction force of the pivot bearing 24. In the cylinderconstruction in accordance with the present invention, the diameter ofthe flange may be sufficiently increased by the effective use of theportion above the upper surface of the upper cylinder 13 which is a deadspace. Thus the compactness of the device is not lost.

The method of the present invention is effective or advantagesespecially in the case wherein the upper cylinder assembly for VTRwherein the accuracy of the height of the head 19 must be less than afew microns is applied to a portable VTR whose position always changes.

In the preferred embodiment of the present invention, that is, in thecase of the pivot suction system (A), the depth of the spiral grooves aswell as the thrust clearance were 25μ and the thrust diameter was 15 mm.A suitable suction force of F=400 grams was obtained. This, even whenthe device was maintained in an inverted position, the sufficient usewas permitted (the satisfactory operations were ensured).

The leading end of the pivot bearing which was used in the preferredembodiment and which is a supporting member, is in the form of a sphereso that the contact portion becomes a point contact, whereby the torquemay be decreased.

Furthermore, the supporting member may be formed with a combination of,for instance, a conical surface, a groove V-shaped in cross section, asphere of a small diameter and a plane.

In the preferred embodiment of the present invention, the thrust bearing42 is formed at the upper end of the stationary shaft 21 so that thepresent invention has succeeded to provide the construction which maymake full effective use of a portion located inside the inner wall ofthe upper cylinder 13 without any waste in space.

The thrust bearing 42 with the above construction has the followingeffects, features and advantages:

(i) The precession may be prevented.

(ii) The function as an axial stopper when the upper cylinder 13 isremoved in case of the exchange of the head 19 and the assembly of thedevice.

When the unbalanced quantity of the rotating part exists, the precessionresults. When the pivot suction system is for instance employed, theleading end of the pivot bearing 24 formed at the leading end of thestationary shaft 21 becomes a pivot of precession.

The device of the present invention is featured in that the variationsin the position of the head 19 due to the precession may be reduced to aminimum.

One of the reasons is the position of the pivotal point.

Let the axis of the stationary shaft 21 be the Z-axis and let constructthe X- and Y-axes in the plane perpendicular to the Z-axis andcontaining the pivotal point which is the origin. According to thepresent invention, the moments about the X- and Y-axes may be reduced toa minimum.

The smaller the moments about the X- and Y-axes, the smaller theinclination (of the pivot) to the vertical becomes. The relation betweenthe precession and the pivotal point may be intuitively understood whenone compares a top with a high center of gravity with a top with a lowcenter of gravity. The construction of the device in accordance with thepresent invention corresponds to that of a top with a low center ofgravity which is stable.

In the device of the present invention, the pivotal point of rotationmay be located within the upper cylinder. In addition, the thrustbearing 42 which is located adjacent to the pivotal point has the actionof compensating the inclination of the axis of the rotating member sothat the precession may be effectively prevented and consequently thevariations in position of the head 19 may be reduced to a minimum. Theflange of the thrust bearing 42 has a function of a stopper between therotating member and the stationary member (the stationary shaft 21) inthe case of the exchange of the upper cylinder 13.

FIG. 5 shows a thrust spiral groove plain bearing.

A flat surface 49 is formed with alternative grooves 50 and ridges 51which are equiangularly spaced apart from each other.

When the thrust bearing rotates in the direction indicated by the arrowD, the lubricating liquid on the thrust flat surface 49 is exerted withthe force which causes the lubricating fluid to flow in the directionindicated by the arrow E.

On the other hand, when the thrust bearing rotates in the direction C,the lubricating fluid is exerted with the force in the direction F.

If the thrust plain bearing is held stationary while a surface inopposed relationship with the flat surface 49 rotates, the direction ofthe force acting on the fluid is reversed entirely.

Same is true for the radial spiral groove bearings.

The ridges and grooves of the spiral grooves may be defined bylogarithmic spirals or the like. The effects of the pumping and wedgeactions of the fluid bearings may be equally attained by the use ofother cuves or straight lines.

Therefore, the term "spiral grooves" may interchangeably refer to "anysuitable groove shapes adapted to produce both the pumping and wedgeactions" in this specification.

The grooves used in the hydraulic bearings are provided in order toattain a higher degree of hydraulic lubrication. Of course, even withoutthe provision of grooves, the construction of the cylinder of thepresent invention may attain sufficient effects of hydrauliclubrication.

In addition to the spirally grooved bearings which are imperfectlyround, there are multi-arc bearings, stepped bearings and so on. Theymay be applied to the present invention in order to attain effectivehydraulic lubrication.

For instance, a multi-arc bearing may be formed at the radial bearingportion 39 shown in FIG. 3 and stepped bearings may be formed at theupper and lower surfaces of the thrust bearing.

FIG. 6 shows one of the preferred embodiments or applications of thepresent invention. This is the preferred embodiment wherein theconstruction is further made compact and the pivot bearing carries thethrust load acting on the upper cylinder 13.

52 is a radial spiral bearing a; 53, a radial spiral bearing b; 54, aclearance; and 48, a beveled portion. The radial bearings a and b aredirectly formed in the stationary shaft 21 in such a way that the spiralgrooves are opposite in direction. Due to the pumping action of thespiral grooves, the lubricating oil is exerted with the force indicatedby the arrow in FIG. 6.

As a result, a negative pressure is produced in the clearance 54 so asto establish the pressure equilibrium in such a way that the fluid whichtends to flow in the direction indicated by the arrow may be retracted.

Since a negative pressure is generated in the clearance 54, the uppercylinder is exerted with the force in the direction F in FIG. 6 due tothe difference between the atmospheric pressure and the negativepressure in the clearance 54, and this force balances the verticalresistance from a point on the pivot bearing 24.

Thus, the upper cylinder receives a thrust support and may revolve aboutthe stationary shaft 21 in a stabilized manner. In this case, aspherical projection may be formed at a portion of the upper cylinder 15in opposed relation with the stationary shaft 21, thereby providing apivot bearing.

When this method is employed, the construction may be made extremelycompact. Opposed to the rolling contact bearings wherein contacts occurat many points, the pivot bearing 24 which may maintain a point contactmay ensure an extremely higher degree of accuracy in rotational speed.

The most serious inherent problem of the pivot bearings is that it isdifficult to carry the thrust loads in both directions. (It cannotsatisfactorily carry the thrust loads acting in both axial directions.)They can control only the direction of the thrust load under the weightof a rotating part.

However, when the method described above is employed, even if the uppercylinder 13 is deviated a very small distance upwardly of the pivotbearing 24, the returning force which is proportional (in magnitude) tothe displacement of the upper cylinder acts so that an extremelystabilized dynamic equilibrium may be maintained.

FIG. 7 shows a further embodiment of the present invention which mayensure a more secure and tight sealing of lubricating oil. Thestationary shaft 21 is formed with spiral grooves 55 and an annulargroove 56 which serves as an oil reservoir. A clearance 57 between thestationary shaft 21 above its annular groove 56 and the rotary sleeve 14serves as a sealing portion. As the rotary sleeve 14 is rotated, thelubricant is pumped up from the oil bottom 58 along the spiral grooves55, whereby the leakage of the lubricant may be prevented.

The effects, features and advantages of the present invention may besummarized as follows:

(1) The rotary head assembly of the present invention may ensure ahigher degree of accuracy in rotational speed.

As described hereinbefore, the present invention makes full use of theadvantages of hydraulic lubrication. As a result, both the radial andthrust loads acting on the upper cylinder of the rotary head assemblymay be carried by the hydraulic bearing in an almost ideal mannerhitherto unattainable by the prior art. In addition, a higher degree ofaccuracy in rotational speed of the upper cylinder may be ensured. Thus,the present invention may substantially overcome the problemsencountered in the prior art rotary head assemblies wherein the uppercylinder is supported by the rolling-contact bearings. Consequently, theperformance of VTR incorporating the rotary head assembly of the presentinvention may be remarkably improved.

(2) The rotary head assembly of the present invention may be made verycompact in size.

The main reason is that the rotary sleeve upon which is mounted theupper cylinder is rotatably mounted through the hydraulic bearing on thestationary shaft. The fact that the rotary sleeve is in the form of apipe also contributes to the reduction in size of the rotary headassembly. The use of the hydraulic bearing can completely eliminate theuse of rolling-contact bearings which have been long used in the priorart rotary head assemblies and which require a large installation space.Thus, according to the present invention VTR with an extremely thinthickness hitherto unattainable by the prior art may be provided.

(3) The mass production may be much facilitated.

The greatest obstacle to the mass production of VTR has been themachining and assembly of the upper cylinder assemblies or the rotaryhead assemblies which must be machined and assembled with a higherdegree of dimensional accuracy. So far with the use of rolling-contactbearings it has been impossible to improve the dimensional accuraciesbeyond certain limits. Since the present invention may completelyeliminate the use of rolling contact bearings, these problems may beovercomed. Thus, the mass production may be facilitated. Furthermore,the component parts may be machined or otherwise fabricated at low cost.The sealing of lubricant may be remarkably improved.

In addition to VTR, the present invention may be equally applied tovarious electrical and electronic devices and equipment such as tapedecks, players, video disks, magnetic drum devices for industrial use,sheet disk recording and reproducing devices and so on, all of whichrequire rotating members which must rotate at a higher degree ofaccuracy in rotational speed for a long service time.

What is claimed is:
 1. A rotary head assembly for a magnetic recordingand reproducing device, comprising:a stationary center shaft having oneend which is securely held in position and a free end, with a circularflange adjacent said free end; a stationary cylinder having acylindrical surface coaxial with said stationary center shaft; a rotarycylinder disposed adjacent to one end of said stationary cylinder forrotation about the axis of said stationary center shaft and having acylindrical surface the diameter of which is the same as that of saidcylindrical surface of said stationary cylinder, said rotary cylinderhaving an axial cylindrical recess therein, said recess including ashoulder portion for receiving said flange; means for driving saidrotary cylinder; a magnetic head mounted on said rotary cylinder; meansfor transporting a magnetic tape around said cylindrical surfaces ofsaid stationary and rotary cylinders at a given speed relative thereto;said stationary center shaft being stationary relative to saidstationary cylinder, the free end of said stationary center shaftextending into said cylindrical recess for supporting said rotarycylinder, said free end having a region of engagement with the bottom ofsaid recess which has an area substantially less than the crosssectional area of said shaft; said rotary cylinder including a rotarysleeve having a sealed upper end defining the bottom of said recess andan open lower end; means for injecting a lubricating fluid into thespace between said rotary sleeve and said stationary center shaft so asto provide a hydraulic bearing means between said stationary centershaft and rotary cylinder, said hydraulic bearing means having (i) aradial bearing portion defined by a peripheral surface part of saidshaft adjacent said flange and relatively remote from said free end, andthe adjacent inner surface part of said rotary sleeve, one of saidsurface parts having a circumferential spiral or helical groove thereinfor forcing said fluid toward the free end of said shaft upon rotationof said rotary sleeve; and (ii) a thrust bearing portion defined by thebottom surface of said recess and the adjacent major surface of saidflange.
 2. A rotary head assembly as set forth in claim 1, wherein saidrotary cylinder is detachably secured to said rotary sleeve.
 3. A rotaryhead assembly as set forth in claim 2, further comprising a pressureseal means for preventing the leakage of lubricating fluid interposedbetween said center shaft and said rotary sleeve.
 4. A rotary headassembly as set forth in claim 2, wherein the bottom off said recesscomprises a cover secured to said rotary sleeve, the surface of saidcover being in opposed relationship with the surface of said flange andhaving radially directed spiral hydraulic bearing grooves therein.
 5. Arotary head assembly as set forth in claim 1, further comprising meansfor restricting relative axial movement between said center shaft andsaid rotary sleeve.
 6. A rotary head assembly as set forth in claim 5,wherein at least one of (i) the surface of said flange remote from saidfree end and (ii) the surface of said recess in opposed relationtherewith, is formed with grooves having relative clearances which varyin the circumferential direction.
 7. A rotary head assembly as set forthin claim 1, wherein at least one of the surfaces defining said thrustbearing portion is formed with radially directed grooves oriented sothat the lubricating fluid in contact with said thrust bearing portionis forced to flow in the centrifugal direction upon rotation of saidrotary cylinder.
 8. A rotary head assembly as set forth in claim 1,further comprising a rotor mounted on said rotary sleeve and a statorsurrounding said rotor, whereby said rotary cylinder may be directlydriven through said rotary sleeve by driving said rotor.